Samsung 172T Intermittent (blinks off, then back on)

[johnmeyer]

This Samsung 172T powers up fine, and looks normal for about 1-2 minutes. Then, it blinks off for a few seconds, then comes back on again. As it continues to operate, it spends more and more time off, and eventually only comes on for a few seconds.

It makes no unusual sounds and, when it is operating, the display is perfect, has normal brightness, and does not flicker.

I have swapped out both the power cable and the cable between the monitor and the computer. Neither change stopped the problem.

This behavior started several months ago, and when it first started, the monitor would blink off for about one second, but only every 5-10 minutes. Thus, it was annoying, but the monitor could still be used. However, the problem slowly got progressively worse, and the monitor is now useless.

After reading several hundred posts in this forum (and elsewhere) I replaced ALL the electrolytic caps (Capxon caps) with 105 degree, low ESR caps (from Digikey, mostly Panasonics). I did not skip any caps. None of the caps were bulging, and when I tested them all after removing from the circuit, all tested OK (I tested capacitance and basic leakage -- I didn't test ESR). The only one that was below nominal capacitance was the 150uF, 450V capacitor. (Just as a side note: It is impossible to get a replacement for this 150uF 450V cap that is as small as the original, so I wired a 100uF and 47uF in parallel. There is just enough space to fit these two caps at right angles to each other inside the heatsink cage.)

Unfortunately, after putting everything back together, the behavior I described above is precisely the same as before (hey, at least I didn't make it worse!).

I'm posting this to see if anyone has other ideas. The duration of the "blink off" outage is pretty consistent, and while I've seen intermittents caused by cold solder joints, I did a pretty thorough inspection, and was unable to detect any obvious cold-solder joints. If it wasn't for all the SMD stuff, I'd just spend fifteen minutes and reflow all the joints.

I'm about ready to throw in the towel on this one, but I thought I'd ask and see if anyone has other ideas. All other components show absolutely no signs of thermal stress.

The board I am working on is absolutely identical to the one shown in the photos in this thread:

Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Thanks!
============
Replacement cap list:
2 - 1000UF 10V ELECT KMG
3 - 470UF 25V ELECT KMG
2 - 1UF 50V ELECT EB
1 - 220UF 25V ELECT EB
1 - 1UF 63V ELECT EB
4 - 47UF 25V ELECT EB
1 - 4.7UF 50V ELECT EB
1 - 100UF 450V ELECT KXG + 47UF 450V KXG
1 - 47UF 50V ELECT

[Krankshaft]

Re: Samsung 172T Intermittent (blinks off, then back on)

Check for cracked solder joints on the power board.

[PlainBill]

Re: Samsung 172T Intermittent (blinks off, then back on)

Check for cracked solder joints on the power board.

I agree, that's about the only remaining cause.

PlainBill

[mockingbird]

Re: Samsung 172T Intermittent (blinks off, then back on)

I have a 172T here, the design is different from the one shown here, but there are three capacitors on the logic board, all they same on them is 18V 220uF "TMQ". I can't find this TMQ series anywhere. Anyone have any ideas?

[Rtech]

Re: Samsung 172T Intermittent (blinks off, then back on)

What is happening to the output voltages when it 'blinks off' ??are they consistent and steady ??.I would also plug the psu ONLY into the mains,plug in the Backlights, apply and external 3Volts Dc into the BL ON connection and see what happens then ?? Wi

This Samsung 172T powers up fine, and looks normal for about 1-2 minutes. Then, it blinks off for a few seconds, then comes back on again. As it continues to operate, it spends more and more time off, and eventually only comes on for a few seconds.

It makes no unusual sounds and, when it is operating, the display is perfect, has normal brightness, and does not flicker.

I have swapped out both the power cable and the cable between the monitor and the computer. Neither change stopped the problem.

This behavior started several months ago, and when it first started, the monitor would blink off for about one second, but only every 5-10 minutes. Thus, it was annoying, but the monitor could still be used. However, the problem slowly got progressively worse, and the monitor is now useless.

After reading several hundred posts in this forum (and elsewhere) I replaced ALL the electrolytic caps (Capxon caps) with 105 degree, low ESR caps (from Digikey, mostly Panasonics). I did not skip any caps. None of the caps were bulging, and when I tested them all after removing from the circuit, all tested OK (I tested capacitance and basic leakage -- I didn't test ESR). The only one that was below nominal capacitance was the 150uF, 450V capacitor. (Just as a side note: It is impossible to get a replacement for this 150uF 450V cap that is as small as the original, so I wired a 100uF and 47uF in parallel. There is just enough space to fit these two caps at right angles to each other inside the heatsink cage.)

Unfortunately, after putting everything back together, the behavior I described above is precisely the same as before (hey, at least I didn't make it worse!).

I'm posting this to see if anyone has other ideas. The duration of the "blink off" outage is pretty consistent, and while I've seen intermittents caused by cold solder joints, I did a pretty thorough inspection, and was unable to detect any obvious cold-solder joints. If it wasn't for all the SMD stuff, I'd just spend fifteen minutes and reflow all the joints.

I'm about ready to throw in the towel on this one, but I thought I'd ask and see if anyone has other ideas. All other components show absolutely no signs of thermal stress.

The board I am working on is absolutely identical to the one shown in the photos in this thread:

Samsung Syncmaster 172N with fried resistors under T1 - No backlight

Thanks!
============
Replacement cap list:
2 - 1000UF 10V ELECT KMG
3 - 470UF 25V ELECT KMG
2 - 1UF 50V ELECT EB
1 - 220UF 25V ELECT EB
1 - 1UF 63V ELECT EB
4 - 47UF 25V ELECT EB
1 - 4.7UF 50V ELECT EB
1 - 100UF 450V ELECT KXG + 47UF 450V KXG
1 - 47UF 50V ELECT

[b700029]

Re: Samsung 172T Intermittent (blinks off, then back on)

This is going to sound strange, but...

I have a pair of 173Ts of about the same age, one of which started exhibiting the exact same symptoms as you described. The other remained perfectly fine. Naturally, I opened the bad one, checked the caps, and they were all fine, so I figured I'd plug it in for a live test. I plugged both of them back into the video card (I'd unplugged both of them) and... the other one started blinking a few minutes after on, the one I'd opened up and was about to probe with a multimeter remained fine... I thought I'd opened the wrong one

On a whim I decided to swap the cables to the two. Both turned on fine, and now the opened-up one started blinking after a few minutes; so there was something about the video signal it wasn't liking. After some more software troubleshooting I found out one of the outputs got set to 76Hz refresh for some reason -- the monitor only supports up to 75Hz. Setting both outputs back to 75 fixed the problem, and I never had to open the thing up in the first place

[johnmeyer]

Re: Samsung 172T Intermittent (blinks off, then back on)

I actually still have this display in the shop, and haven't touched it for the last year. The display refresh is an interesting thing to check because I know that I did change that to a higher refresh many years ago. I'll check that. As for voltages, I didn't check that the last time I had the display apart. I'll do that again. I'm also going to re-solder all the PSU PCB points. I've had TV sets (my old Pioneer 43" CRT) develop thermally sensitive connections on the PCB.

Thanks for the help!